1. Field of the Disclosure
The disclosure relates to a process and apparatus for controlling the gain of laser components including lasers and optical amplifiers. More particularly, the disclosed process and apparatus allow determining the gain of the laser component by monitoring an amplified spontaneous emission (ASE).
2. Prior Art Discussion
FIG. 1 illustrates a typical oscillogram observed during the operation of pulsed lasers at frequencies of about and higher than 500 Hz. As can be seen, two pulses are observed at seemingly the same frequency, which in the illustrated example is about 1 kHz. The detailed analysis of this phenomenon shows that the pulses, in fact are sequential. The existence of sequential pulses may be explained by different gain coefficients before respective consecutive pulses. The non-uniformity of the gain coefficient may be caused by a high population inversion level remaining after a leading pulse.
FIG. 2 is an oscillogram illustrating luminescence 7, which is detected by a photo sensor, and providing the rational behind the pulse doubling phenomenon of FIG. 1. The luminescence level before odd pulses 9 is lower than that one before even pulses 11. Accordingly the population inversion affecting a gain coefficient alternates between lower and higher levels before respective odd and even pulses. Since the odd pulse has lower output energy, the population inversion level remains largely unaffected immediately thereafter as compared to the previously existing level. However, the population grows with a developing even pulse. As a consequence, the gain coefficient associated with even pulses is substantially higher than that one associated with odd pulses. After an even pulse, the population inversion level is substantially depleted and gain coefficient of following odd pulse is relatively low. This fluctuation of the gain coefficient has rather a periodic character and the level of the population thus may grow to prohibitively high levels. As known, high levels of the population inversion may cause undesirable pulsations in laser systems.
The gain is proportional to the optical power of amplified spontaneous emission (ASE). One of the known methods for determining ASE includes the use of optoelectronic sensors, which are operative to measure the integral value of the output optical power, and subsequent comparison of the measured power to a reference value. The method may introduce systematic errors in high power fiber laser systems caused by a variety of uncontrollable losses. One of such losses is represented by the deformation of splices between a signal fiber and a branch fiber carrying a portion of light propagating along an active fiber to the photo sensor due to elevated temperatures. The damaged splice may, in turn, lead to the unstable operation of laser systems and eventually may be the cause of their irreparable damage.
Another known method of measuring an ASE utilizes the pump coupled into an optoelectronic sensor. The method requires frequent calibrating of a power coefficient for the ASE-branched portion. The reliability and cost of such system may be problematic.
Accordingly, there is a need for process stabilizing a population inversion level and minimizing and, desirably, completely eliminating the pulse doubling effect in laser systems by determining the relationship between multiple signals which are selected from different frequency regions of the same ASE power density spectrum.
Still another need exists for a differential apparatus implementing the above-articulated method and operative to monitor the ASE level of the laser system.